While the terminal pitch of each component, for example, a BGA (Ball Grid Array) or a CSP (Chip Size Package) mounted on a printed circuit board has been reduced, the number of terminals of this component is increasing. Since these components have a large number of terminals, they have large outer shapes. Also, since these components have large outer shapes, they are likely to become nonuniform in internal temperature and are likely to warp upon heating when mounting them on a printed circuit board by using a reflow soldering process. Moreover, these components are made of a plurality of materials, including a mold of a package portion and a metal of a terminal portion, and the components are likely to, for example, warp upon heating and cooling when mounting them, depending on the difference in the thermal expansion coefficient between these plurality of materials.
Nowadays, heating related warping is likely for not only components but also a printed circuit board as well. This is due to the recent tendency to reduce the thickness of a printed circuit board, in order to downsize a via hole, which is effective in terms of routing wiring from a narrow-pitch component, and reducing the area of wirings. On the other hand, as the thickness of a printed circuit board is reduced, the board is more likely to warp.
Although components and a printed circuit board warp upon heating in the conventional cases as well, the rate of occurrence of a problem resulting from warpage of the above-mentioned components and printed circuit board upon mounting components by using a reflow soldering process increases. Conventionally, even if a printed circuit board warps, this problem can be solved by increasing the amount of cream solder. However, due to an increase in number of cases wherein both large components and narrow-pitch components with narrow terminal pitches are mounted on the same printed circuit board, the above-mentioned problem cannot be solved simply by increasing the amount of cream solder. This is because increasing the amount of cream solder to provide an advantage in terms of mounting large components, and decreasing the amount of cream solder to avoid a bridge circuit on mounting narrow-pitch components have a trade-off relationship.
As measures against failures in mounting due to warpage as described above, two approaches are possible. The first approach is to take a measure against the warpage of both components and a printed circuit board. Japanese Patent Laid-Open No. 10-107176, for example, discloses a solder bump forming method of partially changing the ball size in accordance with warpage that has occurred upon the manufacture of a BGA or a CSP to adjust the state of connection by soldering, thereby coping with failures in mounting. This prevents the occurrence of failures in mounting by compensating for the amount of warpage of components by changing the size of a solder ball.
The second approach is to partially increase/decrease the amount of cream solder to be applied onto a printed circuit board to compensate for the amount of warpage.
FIGS. 8A to 8C depict views showing the states of cream solder when mounting a component on a printed circuit board 1 by using the conventional reflow soldering process. Referring to FIGS. 8A to 8C, (a) is a top view of the printed circuit board 1, and (b) is a sectional view of the printed circuit board 1. FIG. 8A shows the printed circuit board 1 before cream solder is applied onto it, and FIG. 8B shows the printed circuit board 1 after cream solder 3 is applied onto it. Conventionally, the cream solder 3 is applied onto a wiring pattern 2 in a length Yb equal to that of the wiring pattern 2 of the printed circuit board 1, and the amount of the cream solder 3 applied is controlled by changing a thickness of a metal mask. In FIG. 8B, the cream solder 3 is applied on the wiring pattern 2 in thickness of h. Note that reference numeral 6 denotes a solder resist. FIG. 8C depicts a view showing the printed circuit board 1 applied with solder 4 (the cream solder 3 has been melted by heating) having a thickness h1 after a reflow soldering process.
To partially increase the amount of the cream solder 3, it is necessary to partially change the thickness of a metal mask, that is, to provide a special metal mask. In contrast to this, Japanese Patent Laid-Open No. 2009-10257, for example, discloses a method of changing the thickness of the land of a corresponding wiring pattern of a printed circuit board to change the thickness of cream solder applied onto the wiring pattern, for each component. This method obviates the need to use a special metal mask, and can therefore prevent a rise in cost, and can densely mount components.
However, the former method of taking a measure against the warpage of components and a printed circuit board is imperfect because the measure cannot cope with a large number of components. The measure against warpage cannot be taken for a large number of components with no solder bumps, such as multi-pin connectors, among components to be mounted on an actual printed circuit board. In addition, printed circuit boards made of materials with high melting points, for example, are commonly used, and the use of these printed circuit boards is effective as a measure against warpage but raises the cost.
Also, the latter method of partially changing the thickness of applied cream solder requires a process of manufacturing a special printed circuit board in which the thicknesses of a part of wiring patterns are changed. Although another method of partially changing the thickness of a metal mask used to print cream solder in place of changing the thicknesses of wiring patterns is known, it raises the cost because the metal mask has a special shape.
Moreover, when narrow-pitch components with narrow terminal pitches and large outer shapes are mounted, it is necessary to change the amount of cream solder applied, in accordance with the positions of terminals in one component.
FIG. 1 is a graph showing the warpage value of a component having multi-pin upon heating, and that of a printed circuit board on which the component is to be mounted, upon heating. FIG. 1 shows the longitudinal direction of the component and printed circuit board on the abscissa, and the amount of warpage on the ordinate.
Referring to FIG. 1, reference numeral 101 denotes warpage of a component, reference numeral 102 denotes warpage of a printed circuit board, and reference numeral 103 denotes the total amount of warpage of the component and printed circuit board. The amounts of warpage of the component and printed circuit board are relatively large in the vicinities of their central portions. In this manner, a component and a printed circuit board often warp in opposite directions, so failures in mounting (solder separation) occur in a component with a large total warpage value, or in portions in the vicinity of the central portion of the board. In the future, such property may pose a serious problem upon mounting a larger number of types of components on a printed circuit board at a higher density.